1. Field of the Invention
The present invention relates to an ultrasonic-motor control system which controls the drive frequency of a voltage applied to piezoelectric ceramic elements of an ultrasonic motor.
2. Description of the Related Art
In rotary disc type ultrasonic motors, a stator is composed of an elastic body and piezoelectric ceramic elements which are adhered to the elastic body. The piezoelectric ceramic elements are polarized in the direction of the thickness thereof to thereby cause alternate expansion and shrinkage as voltage is applied. This causes bending oscillation at the surface of the elastic body. As the different voltage phases (SIN and COS waves) are applied to each electrode of the piezoelectric ceramic elements, the resulting waves are combined to form a progressive wave (traveling wave) on the surface of the elastic body. With pressure applied on the rotor, rotational motion in the opposite direction of the progressive wave is generated as a result of frictional force between the stator and the rotor. In this type of ultrasonic motor, it is known that the drive efficiency of the motor improves as the rotational speed becomes maximum if the motor is driven at the resonance frequency thereof.
Accordingly, in the case of driving the ultrasonic motor with the resonance frequency, the drive frequency of the ultrasonic motor is conventionally controlled in the following manner: a starting frequency is set to be sufficiently higher than the resonance frequency of the ultrasonic motor, and subsequently the drive frequency is gradually decreased until it reaches the resonance frequency. Upon reaching the resonance frequency, the drive frequency is fixed, and is finely adjusted so as to maintain the resonance frequency. This control is performed because the resonance frequency (i.e., the optimum drive frequency) of the ultrasonic motor varies due to a variation in the ambient temperature and/or a variation in the load acting on the ultrasonic motor. In addition, the drive frequency of the ultrasonic motor needs to be decreased gradually from a sufficiently high frequency on the assumption that the resonance frequency would shift toward the high frequency side due to some cause.
However, in the case where resonance frequency shifts toward the low frequency side in a normal state or due to some cause, it takes a long time before the ultrasonic motor starts rotating if the sweep operation of the drive frequency of the ultrasonic motor starts from a sufficiently high frequency in a conventional fashion. This becomes a leading cause of a substantial delay in the response of the ultrasonic motor.